Subject: Disabled LocalPlus Controller (LPC) clock on MPC512x

From: Alexander Popov <[email-masked]>
Date: Wed, 26 Nov 2014 14:49:46 +0300

Hello.

My Freescale TWR-MPC5125 board instantly reboots if I touch
any physical address on the LocalPlus Bus (LPB) for the first time
when Linux has already booted.

This effect is reproduced by using /dev/mem or loading a kernel module
which works with any peripherals on LPB.

It took me some time to find out that such crash is caused by
clk_disable_unused() in drivers/clk/clk.c, which disables
LocalPlus Controller (LPC) clock if I don't touch LPB addresses in the
previous initcalls. So starting Linux with clk_ignore_unused bootparam
or inserting dummy LPB reading to some initcall is a temporary fix.

Is it correct to gate LPC clock? If yes, how to avoid the mentioned
crashes properly?

There's a piece of code in arch/powerpc/platforms/512x/clock-commonclk.c
which is doubtful for me:

/*
  * pre-enable those "internal" clock items which never get
  * claimed by any peripheral driver, to not have the clock
  * subsystem disable them late at startup
  */
clk_prepare_enable(clks[MPC512x_CLK_DUMMY]);
clk_prepare_enable(clks[MPC512x_CLK_E300]);    /* PowerPC CPU */
clk_prepare_enable(clks[MPC512x_CLK_DDR]);    /* DRAM */
clk_prepare_enable(clks[MPC512x_CLK_MEM]);    /* SRAM */
clk_prepare_enable(clks[MPC512x_CLK_IPS]);    /* SoC periph */
clk_prepare_enable(clks[MPC512x_CLK_LPC]);    /* boot media */

Does it mean that these clocks should be registered with
CLK_IGNORE_UNUSED flag?

Thanks a lot.
Best regards,
Alexander